And their risk of dying from any kind of heart disease is eight times higher than for those without diabetes, the long-term Danish study also found.

"It is important that healthcare providers are aware that young patients with diabetes have an elevated risk of mortality and that this is mainly explained by an increased risk of sudden cardiac death," said the study's lead author Jesper Svane, a medical student at Copenhagen University Hospital in Denmark.

Sudden cardiac death is caused by malfunctions in the heart's electrical system. It often occurs without warning, according to the American Heart Association.

Dr. James Catanese, chief of cardiology at Northern Westchester Hospital in Mount Kisco, N.Y., said he wasn't surprised to see the link between diabetes and heart disease deaths.

"What was surprising was the amount of increased risk -- a 7 or 8 times higher risk is astounding, particularly in people below age 50," added Catanese, who wasn't involved in the study.

The 10-year study included health information from all Danes between 1 and 35 years old in 2000-2009 and from those 36 to 49 years old in 2007-2009.

Of more than 14,000 people who died, 5 percent had diabetes, according to the study. Almost 500 of them had type 1 diabetes, which is an autoimmune disease. And nearly 200 who died had type 2 diabetes, which is the more common form of the disease and it's generally linked to excess weight.

Overall, the researchers found, people with diabetes had a five times higher mortality rate than people without diabetes.

More specifically, they found, death from heart disease was five times higher in people with type 2 diabetes, and 12 times higher in people with type 1 diabetes. Svane said this may be because type 1 is often diagnosed in childhood, so patients have the disease for a longer period of time.

The research couldn't prove a cause-and-effect relationship, only an association, Svane explained.

But what might cause this association between diabetes and the risk of heart disease?

"Fluctuating blood sugars, abnormal cholesterol and high triglycerides, which are present in many persons with diabetes mellitus, increases the risk of [hardening of the arteries] and coronary heart disease," he noted. This eventually increases the risk of sudden cardiac death or heart failure, he explained.

An additional risk factor for people with type 1 diabetes is a condition called "dead-in-bed" syndrome. This term describes a sudden, unexplained death in a young person with type 1 diabetes, Svane said.

"The underlying mechanism leading to dead-in-bed syndrome remains unknown. However, growing evidence is pointing at both autonomic neuropathy and nocturnal hypoglycemia (low blood sugar) as causes," Svane said. There were six dead-in-bed cases included in the current research.

Autonomic neuropathy is a complication of diabetes that causes nerves that control important body functions -- such as digestion or blood pressure regulation -- to malfunction, the American Diabetes Association says.

So what should people with diabetes do to lessen their risk?

Svane said previous studies have shown that tight control and effective treatment of cholesterol, blood pressure and blood sugar can reduce the risk of heart-disease-related deaths in people with diabetes.

"Pay attention to diet and to a lack of physical activity," he advised. "Exercising and losing weight can prevent or delay the onset of type 2 diabetes, reduce blood pressure and help reduce the risk for heart attack and stroke." And anyone who smokes should quit, he added.

Catanese agreed that aggressive treatment of diabetes, cholesterol and blood pressure is key.

"People with diabetes need to know they're at risk of heart disease death right now, and they need to take care of their heart right from the onset of diabetes," he added.

The study was scheduled to be presented Monday at the American Heart Association annual meeting, in Anaheim, Calif. Findings presented at meetings are typically viewed as preliminary until they're published in a peer-reviewed journal.